Extrusion heads of the general type referred to hereinbefore are known and are used for making products such as, for example, tread strips for tires for automotive vehicles. To produce tread strips, it is necessary for three or more different types of mixtures to be combined in the extruder head. This is because the strip comprises a tread surface portion formed from a highly wear-resistant material, lateral portions made of highly compressible material, a so-called lower or base portion which forms the bottom layer of the tread surface and is usually formed from a softer mixture than the tread surface, and a so-called pad mixture which ensures good adhesion of the strip to the carcase of the tire.
Accordingly, up to four mixtures, which have markedly different viscosities, must be brought together in a single extrusion head to form the tread strip profile. Moreover, the mixture must flow through the flow channels defined in the extrusion head at substantially identical flow rates.
In British Patent Specification No. 2 131 734 A, there is disclosed an extrusion head suitable for producing flat profiles from three different types of mixtures. Such a head is used in conjunction with a plurality of extruders, each extruder extruding a different material, which materials are to be combined into a single profile by the extrusion head. Such a head comprises a central portion which is fixedly connected to the extruder and respective upper and lower portions which are pivotable with respect to the central portion. Between these portions of the head, insert members are located. These insert members and appropriate surfaces of the central, upper and lower head portions define flow channels for the materials being extruded.
It is necessary for the head portions to be pivotable so that the insert members can be replaced or cleaned. This is necessary if the extrusion process is being modified so that a different rubber or elastomeric mixture is to be used in one or more of the extrusion devices.
In addition, it is necessary for the head portions to be pivotable away from one another so as to permit access to the screws of the various extrusion devices. This is because the screws require cleaning from time to time and, occasionally, require replacement.
Primarily, however, the head portions are made pivotable towards and away from one another to permit interchange or replacement of the insert members. This is necessary to provide different flow channels in the head and other templates may also be incorporated therein to achieve an optimum configuration for the flow channels. Moreover, by changing the insert members, different profiles are produced. In practice, this is effected relatively frequently because the extrusion apparatus may be required to produce tread strips for a wide range of tires such as different-sized tires for saloon cars, commercial vehicle tires and specialized tires for vehicles such as earth-moving machines.
It is therefore highly desirable that the insert members can be cleaned and/or interchanged very rapidly.